The present invention relates to an optical disc having two or more of recording layers including a recording layer from which data is read by, for example, a blue laser and another recording layer from which data is read by, for example, a red laser and a method of producing such an optical disc.
Optical discs have a large storage capacity and are for use in non-contact reproduction, and hence widely used as read-only type discs, such as, CD and DVD (Digital Versatile Disc)-Video, write-once type discs, such as, CD-R and DVD-R, and also rewritable type discs, such as, CD-RW, DVD-RAM and DVD-RW.
One type of an optical disc in widespread use at present is DVD in compliance with the DVD-Read Only Memory specifications (referred to as the DVD-ROM specifications, hereinafter). Another type of an optical disc which will probably be popular in the near future is a next-generation high-density optical disc that is a Blu-ray disc (BD) in compliance with the Blu-ray Disc Read-Only Format specifications (referred to as the BD-ROM specifications, hereinafter). It is expected that DVD and BD will coexist for a certain period of time.
Under such expectations, disc manufacturers have to produce both DVD and BD for the same video contents, such as movies, which is inefficient in cost performance. Moreover, users may have a trouble with choice, for example, whether to buy a DVD for a movie or a BD player and wait for a BD for that movie.
Such a problem could be solved if there is an optical disc having a DVD-recording layer and a BD-recording layer in compliance with the DVD and BD specifications, respectively.
Most DVDs now available on the market is a dual-layer type with two recording layers for higher storage capacity. It is preferred for such a dual-layer disc to have two DVD-recording layers as well as a BD-recording layer.
Such an optical disc may have a BD-layer structure with a BD-recording layer and a DVD-layer structure with two DVD-recording layers laminated in order on a transparent layer (via which a laser beam is incident). Data stored in each recording layer is read by a laser beam incident from the BD-layer structure side via the transparent layer.
Such an optical disc, however, have to be produced to meet both DVD- and BD-ROM specifications for reproduction of DVD-quality videos by a DVD player and also BD-quality (hi-vision level) videos by a BD player.
In reproduction from the DVD-recording layers of the DVD-layer structure, a laser beam is incident to the DVD-recording layers after passing through the BD-layer structure. Thus, a BD-reflective film that is formed between the transparent layer and the BD-recording layer and constitutes the BD-layer structure with the BD-recording layer is required to meet both of the DVD and BD specifications on reflectivity, which is, however, very difficult.
A higher reflectivity for the BD-reflective film of the BD-layer structure causes decrease in reflectivity on the DVD-recording layers, which lowers video quality in reproduction from the DVD-recording layers. In contrast, a lower reflectivity for the BD-reflective film for higher reproduction performance from the DVD-recording layers causes difficulty in reproduction from the BD-recording layer.
WO00/65584 teaches the thickness range from 8 nm to 20 nm for such a reflective film in a BD-layer structure. This is an appropriate thickness range for the reflective film in a BD-layer structure of an optical disc having a single recording layer with no DVD-layer structure. Thus, there is no discussion on reflectivity for DVD-recording layers.
As discussed above, there is one requirement for an optical disc having at least two DVD-recording layers and one BD-recording layer. It is about the reflective film formed between the BD-recording layer and the transparent layer via which a laser beam is incident in reproduction. In detail, this particular reflective film must meet both of the DVD and BD specifications for its reflectivity.